1. Field of the Invention
The present invention relates to a post-processing apparatus, a control method therefor, and a post-processing system that are used in an image forming apparatus such as a copying machine and a laser beam printer.
2. Description of the Related Art
Up to now, in an image forming apparatus such as a copying machine, a post-processing apparatus such as a finisher is connected to an image forming apparatus main body to provide various post-processes required by a user such as a sheet-bundle deliver process and a staple process.
The post-processing apparatus receives image-formed sheets, which are delivered one by one from an image forming apparatus, to obtain a sheet bundle by causing the plurality of sheets to overlap each other. The post-processing apparatus includes an intermediate tray which is used for executing a staple process with respect to the sheet bundle, and a stack tray which receives the sheet bundle produced on the intermediate tray and delivered onto the stack tray.
In addition, the post-processing apparatus executes sheet alignment in a transport direction on the intermediate tray every time the sheet is delivered onto the intermediate tray. Further, when sheets corresponding to a sheet bundle are delivered onto the intermediate tray, in addition to the sheet alignment, a sheet-bundle delivery process onto the stack tray is performed after the staple process or the like has been applied. After the sheet-bundle delivery process has been executed, it is possible to deliver another sheet onto the intermediate tray.
Accordingly, it is necessary to adjust a delivery timing of another sheet by considering time required for completing the sheet bundle delivery process.
In order to adjust the delivery timing, first, there is a method in which the image forming apparatus adjusts an image forming timing for each sheet according to time required for performing a variety of processes, thereby adjusting a delivery time of each sheet to be delivered onto the post-processing apparatus from the image forming apparatus. However, when the method is adopted, it is difficult to uniform time intervals required for executing image formation with respect to sheets, which results in lowering productivity.
Second, there is a method (i.e., buffering method) in which, after the sheets delivered from the image forming apparatus are received by the post-processing apparatus, the sheets are allowed to stand by until a predetermined number of sheets are accumulated halfway in a transport path to be delivered onto the intermediate tray, and when the predetermined number of sheets are accumulated in the transport path, the sheets are simultaneously delivered onto the intermediate tray in a state where a plurality of sheets overlap one another.
In this case, in the image forming apparatus, image formation with respect to the sheet and sheet delivery to the post-processing apparatus may be executed at predetermined time intervals irrespective of the time required for performing the post-process. As a result, it is possible to prevent the productivity from being lowered.
As the buffering method, for example, Japanese Patent Application Laid-Open No. 2000-351522 discloses a method in which leading edges of two sheets are allowed to abut against a stopper or a nip of a roller pair to cause the two sheets to overlap each other, to thereby transport the sheets to the intermediate tray. In addition, as disclosed in Japanese Patent Application Laid-Open No. 2000-327208, there is a well-known method in which sheets are allowed to stand by until a plurality of sheets are accumulated in a branch path provided for the sheets to stand by without allowing edge portions of the plurality of sheets to abut against a stopper member, to thereby guide the sheets onto the intermediate tray while the plurality of sheets are caused to overlap one another.
When a sheet bundle (i.e., a plurality of sheets) is delivered onto the intermediate tray to perform the sheet alignment in a transport direction (i.e., alignment in a vertical direction) on the intermediate tray by adopting those buffering methods, it is necessary to allow the edge portions of the sheets, which overlap one another, to reliably abut against the stopper (i.e., reference member) for aligning the sheets. When there is even a single sheet that is not abutted against the stopper, the sheets may not be aligned.
A post-processing apparatus 1 shown in FIG. 34 is provided with a buffering part 2 with respect to a plurality of intermediate trays 3 and 4. Each of the intermediate trays 3 and 4 is provided with a stoppers 3a and 4a. 
FIGS. 35A and 35B are structural views each showing a partially enlarged part of the post-processing apparatus 1, and showing a state where a sheet bundle constituted of three sheets, that is, sheets P1, P2, and P3, is outputted to an intermediate tray 3 or an intermediate tray 4 from the buffering part 2. In FIG. 35A, the sheet bundle is constituted by causing the three sheets P1, P2, and P3 to overlap one another so that the sheet P1 is ahead of the sheet P2, and the sheet P2 is ahead of the sheet P3. On the other hand, in FIG. 35B, the sheet bundle is constituted by causing the three sheets P1, P2, and P3 to overlap one another so that the sheet P3 is ahead of the sheet P2 and the sheet P2 is ahead of the sheet P1.
With respect to each sheet bundle outputted to the intermediate trays 3 and 4, in a case where a lowermost sheet (i.e., sheet in contact with the intermediate tray) first abuts against stoppers 3a and 4a, the sheets sequentially abut against the stoppers 3a and 4a by their own weight in the order from the bottom. In other words, it is possible to execute sheet alignment in the sheet transport direction. Meanwhile, when an uppermost sheet first abuts against the stoppers, the sheets subsequent to the uppermost sheet and the lowermost sheet cannot abut against the stoppers by their own weight because a predetermined friction force acts on the sheets. In this case, it is impossible to execute the sheet alignment in the sheet transport direction.
In FIG. 35A, with regard to the sheet bundle outputted to the intermediate tray 4, the lowermost sheet P1 first abuts against the stopper 4a, thereby making it possible to execute the sheet alignment in the sheet transport direction. On the other hand, with regard to the sheet bundle outputted to the intermediate tray 3, the uppermost sheet P3 first abuts against the stopper, so it is impossible to execute the sheet alignment in the sheet transport direction.
In FIG. 35B, with regard to the sheet bundle outputted to the intermediate tray 3, the lowermost sheet P1 first abuts against the stopper 3a, thereby making it possible to execute the sheet alignment in the sheet transport direction. On the other hand, with regard to the sheet bundle outputted to the intermediate tray 4, the uppermost sheet P3 first abuts against the stopper, so it is impossible to perform the sheet alignment in the sheet transport direction.
As described above, in the case where the post-processing apparatus includes one buffering part with respect to a plurality of intermediate trays, when the same buffering method is carried out on all of the intermediate trays, it may be difficult to reliably perform the sheet alignment in the sheet transport direction.
Further, in a case where the post-processing apparatus includes a plurality of intermediate trays, when the buffering part is provided for each of the intermediate trays, a manufacturing cost of the post-processing apparatus is increased.